Invasive Congeners Are Unlikely to Hybridize with Native Hawaiian Bidens

American Journal of Botany 100(6): 1221–1226. 2013.

I NVASIVE CONGENERS ARE UNLIKELY TO HYBRIDIZE WITH NATIVE HAWAIIAN B IDENS (ASTERACEAE)1

M ATTHEW L. KNOPE 2,6 , R ICHARD J. PENDER 3 , D ANIEL J. CRAWFORD 4 AND A NIA M . W IECZOREK 5

2 Department of Geological and Environmental Sciences, Stanford University, 385 Serra Mall, Stanford, California 94305 USA; 3 Botany Department, University of Hawaii at Manoa, 3190 Maile Way, Honolulu, Hawaii 96822 USA; 4 Department of Ecology and Evolutionary Biology, and the Biodiversity Institute, University of Kansas, Lawrence, Kansas 66045 USA; and 5 Department of Tropical Plant and Soil Sciences, University of Hawaii at Manoa, 3190 Maile Way, Honolulu, Hawaii 96822 USA

• Premise of the study: Invasive plant species threaten native plants in multiple ways, one of which is genetic assimilation through hybridization. However, information regarding hybridization between related alien and native plant species is generally lacking. In Hawaii, the invasive Central American species Bidens pilosa and Bidens alba have colonized natural areas and often grow alongside the native Hawaiian Bidens species, a clade representing an adaptive radiation of 27 endemic taxa, many of which are threatened or endangered. • Methods: To assess the risk of hybridization between introduced and native Hawaiian Bidens (which will readily hybridize with one another), we undertook crosses in cultivation between the invasive species and nine native Bidens taxa. • Key results: The majority of the crosses formed no viable seed. Although seed did mature in several of the crosses, morphological screening of the resulting seedlings indicated that they were the result of self-pollination. • Conclusions: This result suggests that B. alba and B. pilosa are incapable of hybridizing with these Hawaiian Bidens taxa. Further, we found that B. alba in Hawaii was self-compatible, despite self-incompatibility throughout its native range, and that the tetraploid species B. alba and the hexaploid species B. pilosa were cross-compatible, although pollen fertility was low.

Key words: adaptive radiation; Bidens ; conservation; endangered species; endemic species; gene ﬂ ow; introgression; invasive species; islands; threatened species.

Biotic invasions are a major threat to native biodiversity could lead to extinction of the native species. Assimilation oc- throughout the world (Mack et al., 2000; Cadotte et al., 2006; curs when highly fertile hybrids backcross to parental species, Lövei et al., 2012), and invasive plants are commonly cited potentially resulting in the loss of one or both of the parental as examples of alien species threatening native species (e.g., species through time. This risk is potentially heightened if the Kueffer et al., 2010 ; Young et al., 2010 ). There have recently native populations are small relative to the alien populations been extensive efforts to identify the traits (e.g., life history, (Wendel and Percy, 1990; Haber, 1998; Daehler and Carino, breeding system, prior evolutionary history) that predict inva- 2001). In addition, hybrid offspring that suffer lower fi tness siveness (e.g., Lloret et al., 2005; Abe et al., 2011; Castro-Díez than either parent, through reduced vigor, fertility, or abortion et al., 2011; Kaiser-Bunbury et al., 2011) and the factors that of embryos, may adversely affect the native species, particu- account for differences in the invasiveness of species (Kueffer larly in small populations due to gametic wastage and reduced et al., 2010 ). One of the many processes by which invasive spe- seed set ( Daehler and Carino, 2001 ). cies pose threats to native species is interspecifi c hybridization The Hawaiian Islands fl ora may be particularly susceptible to (Levin et al., 1996; Daehler and Carino, 2001). If native species the adverse affects of hybridization with invasive species. Ap- are cross-compatible and interfertile with alien species, and the proximately 89% of the 956 fl owering plant species native to fi tness of the resulting hybrids is equal to or exceeds their par- the Hawaiian Islands are endemic (Wagner et al., 1999) and ents in some habitats, genetic assimilation and competition hybridization with alien congeners is known to occur. For example, naturally occurring hybrids between native-alien Rubus

1 (Randell et al., 1998) and native-alien Portulaca (Kim and Carr, Manuscript received 12 January 2013; manuscript accepted 11 March 1990 ) and both natural and greenhouse hybrids between native- 2013. M.L.K. gratefully acknowledges T. Fukami for support and advice; alien Gossypium species ( Stephens, 1964; Dejoode and Wendel, W. Haines, S. Hinard, K. Kawakame, J. Knope, T. Kutynina, K. McMillen, 1992) have been documented in Hawaii. Daehler and Carino K. A. McMillen, D. Okamoto, H. Oppenheimer, and A. Yoshinaga for assis- (2001) estimate that a total of 59 genera, consisting of 176 native tance in the fi eld; and the Ecology, Evolution, and Conservation Biology fl owering plant species in Hawaii, are potentially threatened with program at the University of Hawaii and the Dept. of Biology at Stanford hybridization by alien congeners. University for fi nancial support. L. Kerr, A. Lehman, S. Lucas, C. Okazaki, One of the largest lineages of Hawaiian fl owering plants and S. Walsh provided assistance with growing plants and undertaking is in the genus Bidens (family Asteraceae or Compositae, tribe crosses at UH. R.J.P. gratefully acknowledges the fi nancial support provided Coreopsideae), including 19 species and eight subspecies en- by Fulbright New Zealand and the John R. Templin Scholarship. K. Sadler demic to Hawaii, resulting from a single colonization event assisted with cultivation of plants at KU. Lastly, we thank P. Feinberg for image preparation, and L. Castillo Nelis, L. Frishkoff, T. Fukami, and two ( Ganders et al., 2000 ; Knope et al., 2012 ). Despite their coloni- anonymous reviewers for comments. zation of the archipelago within the last ~3 million years (Knope 6 Author for correspondence (e-mail: [email protected]) et al., 2012 ), more morphological and ecological diversity exists in the Hawaiian lineage than in the remaining ~300 species doi:10.3732/ajb.1300018 of Bidens distributed over fi ve continents ( Ganders and Nagata,

1984; Crawford et al., 2009; and see Fig. 1). The Hawaiian taxa and Lim, 1970) and will readily hybridize with one another are considered to be one of the best examples of adaptive radia- ( Ganders and Nagata, 1984 ). The Bidens pilosa species com- tion in the Hawaiian ﬂ ora ( Carr, 1987 , and nine taxa are cur- plex, which has a center of diversity in its native range in Mexico rently of conservation concern, including both threatened and ( Ballard, 1986 ), is an invasive species now common in Hawaii, endangered taxa (Wagner et al., 1999). Given the great ecologi- as it is in subtropical and tropical regions worldwide (IUCN cal diversity of the Hawaiian Bidens ( Ganders and Nagata, ISSG, 2011). This species has a high score on the Hawaiian 1984; Crawford et al., 2009), any threat to the Hawaiian taxa Weed Risk Assessment (HI-WRA, 2011), which is based on would impact a disproportionately high level of biological di- approximately 50 attributes that characterize invasive spe- versity relative to the overall number of taxa in the genus. Two cies. Bidens pilosa is morphologically variable, but the com- widely distributed species, Bidens pilosa and Bidens alba , have mon weedy form, like the native Hawaiian Bidens species, is been introduced into Hawaii, and they occur as the sister group hexaploid with a chromosome number of n = 36 (Ballard, 1986). in the same large clade as the native Hawaiian Bidens species The species is self-compatible and exhibits autonomous autog- ( Ganders et al., 2000 ; Kimball and Crawford, 2004 ; Knope et al., amy with high seed set (Sun and Ganders, 1990; Grombone- 2012). The close phylogenetic relationship between the native Guaratini et al., 2004). Bidens alba, a species that is closely and alien Bidens species suggests the potential for hybridiza- related to B. pilosa (Ballard, 1986; Ganders et al., 2000; Knope tion in Hawaii. et al., 2012 ), is also invasive in Hawaii, and like B. pilosa , can The native species grow from sea level to over 2200 m in be found growing side-by-side with native Hawaiian Bidens elevation and occur in habitats ranging from semideserts to rain- species (Gillett, 1972). We have also encountered ﬁ ve native forests (Ganders and Nagata, 1984). About half of the Hawaiian Bidens taxa growing immediately adjacent to one or the other native taxa are gynodioecious (dimorphic breeding system in of the introduced Bidens species in various locations through- which female individuals coexist with hermaphroditic indi- out Hawaii (M. L. Knope and R. J. Pender, personal observa- viduals in populations), a form of sex expression not known tions). However, B. alba differs from B . pilosa in being tetraploid elsewhere in Bidens ( Sun and Ganders, 1986 ). The Hawaiian ( n = 24) and has previously been considered self-incompatible species are self-compatible and those that have been investi- ( Ballard, 1986 ). gated have a mixed mating system ( Ritland and Ganders 1985 ; The purpose of the current study was to determine whether Sun and Ganders, 1986). All of Hawaii’s native Bidens species experimental hybrids between native Hawaiian Bidens species are hexaploid with a gametic chromosome number of 36 ( Gillett and B. pilosa or B. alba could be synthesized and thus provide

Fig. 1. Representative invasive and native Hawaiian Bidens taxa used in experimental crosses: (A) B. pilosa (Central American species now invasive in Hawaii) (photo credit: G. D. Carr); (B) B. alba (Central American species now invasive in Hawaii) (photo credit: J. C. Knope); (C) Hawaiian native B. torta with C. W. Morden in foreground for scale (photo credit: G. D. Carr); (D) Hawaiian native B. cosmoides (photo credit: C. H. Lamourex); (E) Hawaiian native B. menziesii (photo credit: C. H. Lamourex); (F) Hawaiian native B. sandvicensis confusa (photo credit: M. L. Knope); (G) Hawaiian native B. mauiensis (photo credit: G. D. Carr); (H) Hawaiian native B. sandvicensis sandvicensis (photo credit: M. L. Knope); (I) Hawaiian native B. hillebrandiana (photo credit: Forest and Kim Starr); (J) Hawaiian native B. micrantha (photo credit: K. Magnacca); and (K) Hawaiian native B. amplectens (photo credit: G. D. Carr). June 2013] KNOPE ET AL.—HAWAIIAN BIDENS AND HYBRIDIZATION 1223 an assessment of the potential threat that these alien species old BioMedical building [four plants]). To preclude the possibility of low pol- pose to native Bidens species. Although there are prior reports len fertility limiting self seed set, we checked pollen viability of all selfed plants on potential hybridization between native Bidens species and and found it to exceed 90%. In crosses undertaken at UH, once the majority of fl orets in a capitulum had B . pilosa, the evidence is ambiguous and confl icting ( Gillett, passed to the female phase (all Hawaiian species are protandrous), they were 1972; Sun and Ganders, 1990; Daehler and Carino, 2001). Fur- supplemented with copious pollen from one of the two alien species. To achieve ther, given the rarity and vulnerability of the native species and this, we rubbed capitula of the male alien donor species onto the capitula of the the highly invasive nature of the alien species, a more defi ni- native species. The pollen of the alien species conspicuously colors the style tive assessment of the potential for hybridization is warranted. branches of native species, which ensured the application of adequate pollen Gillett (1972, p. 481) , in a study on cross compatibility between loads. Crosses undertaken at KU used the same technique as that at UH for making some crosses and, in addition, employed an approach in which the ca- Hawaiian and a Marquesan species of Bidens , commented that pitula of the alien species were rubbed with the native species for several con- “experimental crosses between B . pilosa and Hawaiian species secutive days. This presumably would result in pollen competition between the have all failed.” Gillett (1972) further stated that while B . pilosa native and alien species, assuming some cross compatibility between them. In and several Hawaiian species occur together in nature, there is Bidens cosmoides, the styles are elongated and foreign pollen is easily placed no evidence of hybridization between them. Yet, Gillett (1972) on the style branches without pollen contamination from the female parent. did not indicate which Hawaiian species he attempted to cross To assess each potential alien-native Bidens parental cross, we harvested mature achenes resulting from crosses and sowed achenes from one (randomly with B . pilosa, nor present any other data. Sun and Ganders chosen) capitulum. Seed viability was determined by germination tests at KU (1990, p. 140) stated that all crosses between B . sandvicensis by placing all the achenes from each cross (if fewer than 50 achenes), or a sub- and B . pilosa failed to set seed and further commented that sample of 50 achenes from crosses that produced more than 50 achenes, on “there is no evidence that any introduced species of Bidens in moistened fi lter paper in Petri dishes. Seedlings were transplanted to soil and the Hawaiian Islands ever hybridize with the endemic species.” grown to fl owering in the greenhouse. However, differences between dark, However, Daehler and Carino (2001) listed Bidens as one of the well-fi lled fruits (indicating presence of an embryo) and those that were shriveled and lighter colored (indicating no embryo present) were immediately ap- Hawaiian groups worthy of study for possible hybridization parent. Seeds that did not germinate were dissected and proved to lack embryos. with alien congeners. This listing was based on observations by Due to limited greenhouse space, 25 or fewer seedlings were grown to fl ower- local botanists that natural hybrids between the two may occur, ing to determine their parentage. However, it was determined that seedlings of but more recently C. Daehler (University of Hawaii, Manoa, the parental species are clearly distinguishable, based on vegetative characters, personal communication) expressed some uncertainty about at an early stage. Pollen viability of the parental and of the fi rst generation off- whether the possible hybrids were between native and alien spring was inferred by observing at least 200 grains stained with lactophenol aniline blue (Kearns and Inouye, 1993) for B. alba and B. pilosa. The large, Bidens species. Additionally, as far as we are aware, there are plump, darkly staining viable pollen grains were readily distinguishable from no prior reports on attempted crosses between native Bidens the inviable shriveled, lightly to negligibly stained pollen grains. species and B . alba .

RESULTS MATERIALS AND METHODS With only two exceptions, no viable seed set occurred in Achenes (dry fruits) were collected in the fi eld or from botanical gardens in crosses made at UH in which alien pollen was placed on style Hawaii from August 2008 through May 2009. Species identifi cations of adult plants bearing achenes were made by morphological examination of species branches of the native species while fl orets were in the female diagnostic characters ( Wagner et al., 1999 ) in the fi eld and were subsequently phase. In the two instances of seed set in native-alien crosses confi rmed by growing each species in cultivation. Achenes were collected from ( Table 2 ), the seeds were germinated and grown to maturity multiple individuals from each population. Seeds were placed in plastic bags in (fl owering and fruiting), and they proved to be the result of the fi eld and then stored at 4° C in the laboratory until they were germinated. selfi ng of the native Bidens taxa. Similarly, only selfed seeds Seeds from populations shown in Table 1 were germinated and the resulting were produced when pollen was supplemented on the capit- progeny grown in greenhouses at the University of Hawaii (hereafter UH) or at the University of Kansas (hereafter KU), and used in crossing trials. Because ula over multiple days for crosses undertaken at KU (Table 2). B. pilosa has been reported as highly self-compatible and self-pollinating ( Sun Generally, 25 progeny from these crosses were grown to fruit- and Ganders, 1990 ), it was used as the pollen parent in crosses involving this ing to document that they were the result of selfi ng and addi- species. Bidens alba was used as both egg and pollen parent in crosses. For the tional seeds (55–85, depending on the number of seeds produced) crosses undertaken at UH, a single individual of each species was used as the were germinated, and morphological comparison with seed- egg donor for each cross. For the pollen donor, multiple fl owers, collected from lings of the two parental species allowed defi nitive identifi ca- a single plant per cross, were used to ensure that there were adequate pollen loads on the stigmas of the recipient fl owers. For the crosses made at KU, again tion of all progeny as selfs. In addition, we noted that all the one plant was used per cross as the egg donor (with the exception of the gyno- invasive and Hawaiian Bidens taxa grown in the greenhouses at dioecious Bidens menziesii , where two female plants were used) and fl orets both UH and KU had growth phenotypes similar to how they from multiple capitula of individual plants were used as the pollen donor in all occur in nature, as found previously by Gillett and Lim (1970) crosses. for some of the Hawaiian taxa. In those crosses made at KU in The native Hawaiian Bidens species have previously been shown to be self which the same method used at UH was employed, no viable compatible (via geitonogamous hand pollination), but autogamous pollination appears rare (Ritland and Ganders, 1985; Sun and Ganders, 1986). For this seed was produced. In the two crosses using female plants of reason, we did not undertake intentional self-pollination treatments for the the gynodioecious B. menziesii, no seed set occurred (Table 2). Hawaiian Bidens taxa. For the introduced species, we performed self-compatibility While B. alba has been reported as self-incompatible through- tests both by bagging fl owers (at UH) and by isolating individual plants in dif- out its native range ( Ballard, 1986 ), our data indicated variable ferent ranges of the greenhouses and manually self-pollinating (at KU). Mature levels of self seed set (5–47%; mean of 26%; n = 4). Selfed seed fruits were then harvested from the open, dry, capitula. For Bidens alba , seed set in all 14 plants of B. pilosa was higher than 90%. Addition- was collected from four plants (Iliau Loop Trail Population, Kokee, Kauai; Table 1 ). One seedling from each of these parent plants was grown to maturity ally, we found evidence that the tetraploid B. alba will cross to assess selfed seed set. The same method was used to assess selfed seed set in with the hexaploid B . pilosa , but the pollen fertility of the off- B. pilosa, except that seeds were collected from parent plants from two separate spring of the four crosses between the two ranged from 31 to populations (1630 Kanalui St., Honolulu [10 plants] and from the UH Manoa 49%. In comparison, the pollen fertility of the parent plants 1224 AMERICAN JOURNAL OF BOTANY [Vol. 100

T ABLE 1. Bidens taxa used (followed by authorities) and collection information.

Taxon Collection locality Collector Date

Bidens alba (L.) DC Iliau Loop Trail, Kokee, Kauai J. Knope, T. Kutynina, K. McMillen, 15-Mar-09 and K. A. McMillen Bidens alba (L.) DC Manoa Valley next to Tropical Plant R. Pender 10-Nov-09 and Soil Sciences greenhouse complex Bidens amplectens Sherff Lyon Arboretum, grown from seed collected on K. Kawakame 16-Mar-09 Kealia Trail, behind Dillingham Air Force Base, Oahu Bidens conjuncta Sherff West Maui, Wailuku District, Waihe’e Valley, back valley H. Oppenheimer 8-Aug-08 south side, above main stream, below “Wall of Tears” Bidens cosmoides (A. Gray) Sherff Hui Ku Maoli Ola Native Plant nursery, Oahu M. Knope and S. Hinard 24-Mar-09 Bidens hawaiensis Gray Amy Greenwall Botanical Garden, Hawaii A. Yoshinaga 6-Jul-08 Bidens hillebrandiana (Drake) Degener Hui Ku Maoli Ola Native Plant Nursery, Oahu M. Knope and S. Hinard 24-Mar-09 Bidens hillebrandiana (Drake) Degener Hui Ku Maoli Ola Native Plant Nursery, Oahu R. Pender 15- Sept-09 Bidens mauiensis (A. Gray) Sherff Maui Nui Botanical Garden R. Pender 19- Jan-10 Bidens menziesii (A. Gray) Sherff Amy Greenwall Botanical Garden, Hawaii A. Yoshinaga Unknown Bidens micrantha kalealaha Kula Forest Reserve, Maui A. Yoshinaga 25-Mar-09 Nagata and Ganders Bidens pilosa L. 1629 Kanalui St. and Kaminaka Dr., Honolulu, Oahu W. Haines 11-Mar-09 Bidens pilosa L. Behind old BioMedical building on UH Manoa M. Knope 13-May-09 campus, Oahu Bidens pilosa L. Manoa Cliff Trail, Oahu M. Waite 1-Nov-09 Bidens torta Sherff Ridge trail between Pahole Gulch and Makua D. Okamoto 25-Mar-09 Valley, Wainae, Oahu Bidens sandvicensis confusa Iliau Loop Trail, Kokee, Kauai J. Knope, T. Kutynina, K. McMillen, 3-Mar-09 Nagata and Ganders and K. A. McMillen Bidens sandvicensis sandvicensis Waahila Ridge, Oahu W. Haines 17-Mar-09 Nagata and Ganders used in these crosses was 95–100% for B. alba and 92–100% Bidens taxa (Gillett, 1972; Sun and Ganders, 1990). Other limi- for B. pilosa . tations of prior studies were lack of quantitative crossing data, the Hawaiian taxa were crossed with B . pilosa alone (and not B. alba ), and the identity of the Hawaiian taxa used in those DISCUSSION crosses was not provided. Thus, the present study confi rms ear- lier reports for B. pilosa and provides the fi rst evidence that the Although it is diffi cult to discount rare hybridization events Hawaiian taxa will likely not cross with B. alba. However, because between otherwise cross-incompatible species, our results sug- this study did not include all of the Hawaiian Bidens taxa, we gest that B. alba and B. pilosa are incapable of hybridizing with cannot rule out the possibility that the introduced Bidens could the Hawaiian Bidens taxa we examined (Table 1). While there hybridize with the Hawaiian taxa not included here. However, have been reports by previous workers suggesting the inability Ganders and Nagata (1984) obtained experimental hybrids in- of the Hawaiian and alien taxa to cross, the reports were pre- volving combinations of all recognized endemic species of sented as asides in papers focused on other topics and other Hawaiian Bidens and found high pollen fertility in all the F1

T ABLE 2. Results of crosses between native and alien species of Bidens in Hawaii. The ﬁ rst species in each cross was the egg parent and the second species in each cross was the pollen parent.

No. viable seeds (total no. Location Taxa used in cross seeds in parentheses) % viable seeds of cross

B. amplectens × B. pilosa 0 (85) 0 KU B. cosmoides × B. pilosa 0 (80) 0 KU B. hillebrandiana × B. alba 1 (47) 2% (1 selfed seedling) UH B. hillebrandiana × B. pilosa 0 (33) 0 UH B. mauiensis × B. alba 0 (21) 0 UH B. mauiensis × B. pilosa 0 (36) 0 UH B. menziesii × B. alba 0 (46) 0 UH B. menziesii × B. pilosa 0 (53) 0 UH B. menziesii (bisexual) × B. pilosa 30 (30) grown to fl owering 100% (30 seeds grown to fl owering, all selfs) KU B. menziesii (female) × B. pilosa 0 (201) 0 KU B. micrantha kalealaha × B. alba 12 (12) 100% (12 seeds germinated, all selfs) KU B. micrantha kalealaha × B. alba 0 (42) 0 UH B. micrantha kalealaha × B. pilosa 16 (140) grown to fl owering 100% (50 of 140 total seeds germinated, all selfs) KU B. sandvicensis subsp. confusa × B. alba 1 (45) 2% (1 selfed seedling) UH B. sandvicensis subsp. confusa × B. pilosa 0 (40) 0 UH B. sandvicensis subsp. sandvicensis × B. alba 0 (32) 0 UH B. sandvicensis subsp. sandvicensis × B. pilosa 0 (120) 0 KU B. torta × B. pilosa 0 (145) 0 KU June 2013] KNOPE ET AL.—HAWAIIAN BIDENS AND HYBRIDIZATION 1225 hybrids. These data, in combination with the crosses attempted Bidens (like all known Asteraceae) likely has a sporophytic in- in this study, suggest that all the endemic Hawaiian species compatibility system, any interference would likely occur at the are unlikely to be cross-compatible with the two alien species. receptive style branches because cross-incompatible pollen Similarly, Daehler and Carino (2001) argue, using Bidens as an would not germinate (e.g., Allen et al., 2011). Also, even though example of a Hawaiian lineage in which all species are cross- the native species are self-compatible, they require fl oral visi- compatible, if an alien congener could hybridize with any one tors for seed set, and increased visitation of alien species by species in a lineage, then it would likely be able to hybridize pollinators could reduce seed set in the native species (Dietzsch with all endemics in that lineage. Further, this work suggests et al., 2011). Therefore, even without gene fl ow between native that the Marquesan Bidens species, which nest within the Hawaiian and alien Bidens , there still exists the possibility for reduced clade ( Knope et al., 2012 ), may also be unable to hybridize seed set in the native species due to the sympatric presence of with either B. alba or B. pilosa, which are also invasive in the the alien species. Marquesas. This may be true for other native Bidens species Crossing data for native and alien congeners are valuable that co-occur with these two invasive species in other areas of even when there is no evidence of the occurrence of natural the world, as they may not be phylogenetically closer to B. alba hybrids. Lack of natural hybrids could be due to the recent in- and B. pilosa than the Hawaiian and Marquesan clade ( Ganders troduction of the aliens or occurrence of native and alien spe- et al., 2000; Knope et al., 2012). However, without a fully re- cies in different habitats. However, eventual spread of alien solved phylogenetic hypothesis or crossing studies that include populations could bring native and alien taxa into contact (as is all species, this evaluation of breeding relationships remains the case for Bidens in Hawaii), and knowing whether they are speculative. cross-compatible is important to formulating conservation strat- In addition to the crossing results between native Hawaiian egies for native plants. and alien Bidens , we found that the recently introduced B. alba is self-compatible in Hawaii, despite self-incompatibility throughout its native range in North and Central America (Ballard, 1986 ). LITERATURE CITED Grombone-Guaratini et al. (2004 , 2005 ) similarly found that although B. alba had only been established in Brazil since 1980, A BE , T . , K . W ADA , Y . K ATO , S . M AKINO , AND I . O KOCHI . 2011 . Alien pol- the populations studied there were also self-compatible. The evo- linator promotes invasive mutualism in an insular pollination system. lution of self-compatibility in plants has long received attention Biological Invasions 13 : 957 – 967 . and colonization of new areas is thought to lead to the evolution A LLEN , A. M. , C. J. THOROGOOD , M. J. HEGARTY , C . L EXER , AND S. J. H ISCOCK . 2011 . Pollen–pistil interactions and self-incompatibility of selfi ng or for selection of self-compatible variants (e.g., Baker, in the Asteraceae: New insights from studies of Senecio squalidus 1955 ; Stebbins, 1957 ; Ward et al., 2012 ; Petanidou et al., 2012 ). (Oxford ragwort). American Journal of Botany 108 : 687 – 698 . The evidence presented here and by Grombone-Guaratini et al. B AKER , H. G. 1955 . Self compatibility and establishment after long dis- (2004 , 2005 ) suggests that B. alba may have either latent ge- tance dispersal. Evolution 9 : 347 – 349 . netic variation for self-compatibility not expressed in its native B ALLARD , R. 1986 . Bidens pilosa complex (Asteraceae) in North and range or the polymorphism exists at such low levels (Barrett Central America. American Journal of Botany 73 : 1452 – 1465 . and Schluter, 2008 ) it has not been previously detected ( Ballard, B ARRETT , R. D. , AND D . S CHLUTER . 2008 . Adaptation from standing ge- 1986 ). Additionally, we found that the tetraploid species B. alba netic variation. Trends in Ecology & Evolution 23 : 38 – 44 . and the hexaploid species B. pilosa were cross-compatible, but B ROCK , M. T. 2009 . Prezygotic barriers to gene fl ow between Taraxacum with low pollen fertility. While these two species have dif- certophorum and the invasive Taraxacum offi cinale (Asteraceae). Oecologia 161 : 241 – 251 . ferent ploidy levels, they are phylogenetically closer to one- C ADOTTE , M. W. , S. M. MCMAHON , AND T . F UKAMI [ EDS .]. 2006 . Conceptual another than to the Hawaiian species ( Ganders et al., 2000 ; Knope ecology and invasion biology: Reciprocal approaches to nature. et al., 2012 ). Springer. Dordrecht, Netherlands. Members of the family Asteraceae commonly have general- C ARR, G. D. 1987 . Beggar’s ticks and tarweeds: masters of adaptive radia- ist pollinators and congeneric species often share the same pol- tion . Trends in Ecology and Evolution 2 : 192 – 195 . linators or suite of fl oral visitors (Ellis and Johnson, 2009; C ASTRO-DÍEZ , P . , O . G ODOY , A . S ALDANA , AND D. M. R ICHARDSON . 2011 . Horsburgh et al., 2011), and in some instances, it has been Predicting invasiveness of Australian Acacia species on the basis documented that native and alien species of Asteraceae have of their native climatic affi nities, life-history traits and human use. the same pollinators ( Brock, 2009 ; Powell et al., 2011 ; Vanparys Diversity & Distributions 17 : 934 – 945 . et al., 2011). Although no pollination studies of alien or native C RAWFORD , D. J. , M. E. MESFIN TADESSE , M . E . M ORT , R. T. KIMBALL , AND C. P. RANDLE . 2009 . Tribe Coreopsideae. In V. A. Funk, A. Susanna, Bidens in Hawaii have been published, it is likely that the same T. F. Stuessy, and R. J. Bayer [eds.], Systematics, evolution and bio- insect pollinators visit sympatric alien and native Bidens spe- geography of the Compositae, 713–730. International Organization of cies. However, as with almost all other Hawaiian plant taxa, Plant Taxonomy, Vienna, Austria. studies of pollinator interactions with native and alien Bidens D AEHLER , C. C. , AND D. A. CARINO . 2001 . Hybridization between na- is an important and understudied issue in need of further in- tive and alien plants and its consequences. In J. L. Lockwood and vestigation. In spite of this shortcoming, our results indicate M. McKinnney [eds.], Biotic homogenization, 81–102. 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